Quote from: gbono on September 02, 2014, 07:12:16 PM
When I lift R63 on the -9V supply I get -9V at ZD5 - someting is loading down the supply - but what?

Try the same check with R14 and R23, should help isolate the problem a bit more.

  Sorry, I seem to have messed up with my interpretation of "breadboard".  I was thinking you meant perfboard (vero or strip board) like the picture Roly posted.
  The radio shack breadboard you have is what I call "protoboard" and is not suitable for a permanent installation (and way more expensive).

Breadboard can be used for prototyping or building.  As this is a one off project you can use the breadboard as the finished product.  If you decide you want to go into production, it would be easier to use printed circuit boards.

The 386 suffix denotes how much voltage the IC can withstand, any version is good for at least 9V, so you don't need to worry about it.

Fuses are optional.  You can't hurt yourself with a 9V battery (unless you really wanted to, and connect wires to it that you pierce your skin with).

  It's hard to believe how hot some of these things run "normally".  But you said it was smoking, which it should not.
  The voltage at the zener will tell you.  If you have 33V there, then the heat is the normal amount.  If there is a fault, you will not have 33V at the zener, but some lower voltage.  (this is assuming the other end of R1 has it's proper approx. 45V)

  So R1 should have 45V at one end and 33V at the other, for a voltage drop of 12V.
With a 330 ohm for R1, it would be running about 1/2W.  So the heat there is probably normal if you have 33V at the zener.
Time to get back to proper troubleshooting of the actual fault.

What is the voltage at the junction of R1 and the zener?

Quote from: TolerBear on August 28, 2014, 01:24:04 PM
There's 2 7832 regulators and 1 7831. .

I think you mean two TIP32 and a TIP31.  Then the 4th device on those individual heatsinks can be seen in the pics as a 7824 regulator.
  So perhaps it is a 33V zener limiting the input voltage to the 7824.

Edit:  looking again at the schematic tolerbear linked: http://spaceryerson.com/pics/RG80-Schematic-1.jpg  it appears to be correct, just missing values.

Quote from: TolerBear on August 25, 2014, 03:37:05 PM

This is the original resistor?  What value did you replace it with?  From the other picture it looks like you installed a 300 ohm?
  I guess it's possible it was a 3.3K ohm, or maybe there is a fault causing the overheating.
  Because of the cap near it that does not have a pair, I'm guessing this is the 24V supply, not the + or - 12V.
  Can you read the numbers off D4?  Looks like 1N47 something, need 2 more digits.

Edit:  I don't know what I did wrong with that quote, just click where it says "quote from..." and it will take you to the pic.

  Yes, the link from reply #37 is working and there are pics there.  Click on the main pic and it sends you to the rest.

Are any of them showing the original discolored resistor you are trying to figure out the value of?

 So this picture is not from the Randall?
http://s18.photobucket.com/user/Jskrapper/media/P1120828.jpg.html

Quote from: BrianS on August 21, 2014, 04:45:44 PM
g1, can you tell me a little more about the heating elements?  I may have a couple different sources for something like that.  Was it luck that they ohmed out correctly, or lucky that you found them?

I was lucky to find them for free, and lucky they were 8 ohms.  I'm not 100% sure they were base board heaters, but was told they were.  I believe they were industrial as they came out of a college. 
  They are about 2 feet long, with ceramic squares stacked and a single thick wire spiraling along the length.  Any modern elements I see pics of are the finned type, which would not allow changing impedance.
  I will try to post a pic, maybe someone will recognize the type.
Meanwhile, thinking about this has made me wonder about stove-top electric elements.  Anybody try these or know if they would be a viable load?

  The ZD3 zener may have just failed.  They usually short when they fail which explains the low resistance reading.
  To be sure there are no other faults on that -15V line, check resistance across the points where the zener used to be.
  A low resistance reading there will mean there are other faults.

  Big power resistors are about the only option.  Most load boxes have them wired up in multiples to get the power rating, and in different combinations with switching for choice of load impedance.  Also fan cooling when dealing with so much power.
  I lucked out with a couple old heating elements from baseboard heaters.  The are wirewound, about 8 ohms each, and good for at least 1KW each.  Because they are wirewound, you can tap off at any point for lower impedances (but lower power handling).

  A couple more things you could try even before you open it, just to make sure:
you mentioned trying other speakers, you need to also try other speaker cables.
Also, try going straight into the aux input with something that puts out a fairly hot signal (some kind of preamp out) to see if the problem still occurs.

  Make sure it has the correct polarity at the connector.  This is critical as the wrong polarity will damage the amplifier.

  For a pair of modern 6L6 or EL34, running around 450V B+, around 40W at the onset of clipping is pretty normal.  Push it into clipping just a little and the power goes up.  I think most of the big manufacturers spec their power at 5% distortion.
Do your power calculations with a meter measuring RMS rather than using the scope numbers and you will see how the power jumps at clipping.